It is known that wrought aluminum alloys have some properties which considerably restrict their fields of application. Specifically, cold-hardened semi-finished products lose strength at comparatively low temperatures after a short time. This loss of strength, to a soft state, takes place as a rule within a very narrow temperature range.
Numerous attempts have been made to explain and influence this behavior of wrought aluminum alloys. For instance, "Aluminium und Aluminiumlegierungen" by Altenpohl (1965) contains about 60 pages of test results and explanations by various authors, who agree that virtually all wrought aluminum alloys which have been tested exhibit a very steep loss of strength between 200.degree. and 300.degree. C. While this loss of strength can slightly be influenced by various measures, an economical and appreciable improvement cannot be effected with the manufacturing methods which are available for normal production of semi-finished products and the like, also in view of economic aspects.
Exceptions are wrought aluminum alloys which contain copper, but these have only a limited deformability and their use is restricted by their low resistance to corrosion.
More recently, it has been attempted, with some success, to improve the loss of strength of wrought aluminum alloys having a relatively high manganese content, by casting the alloy with a much higher solidification rate. High solidification rates can readily be obtained in laboratory work but cannot be achieved in conventional commercial production because special casting methods and equipment are required, which cannot be used with the required safety of operation, at least at the present time, so that the desired improvement of the loss of strength cannot yet be obtained at appreciable production rates and with the desired economy.
The alloy of the invention can be used, inter alia, for exhaust systems for internal combustion engines, tube-bank heat exchangers, ducted plates for solar energy collectors and saline-water reclamation plants, as well as in articles such as kitchen vessels, which can be coated with vitreous enamel and resist corrosion and which must have a high buckling strength in the finished state.
To test materials for the last-mentioned purpose as to their suitability for being coated with vitreous enamel, it is customary to immerse enameled sheet specimens having bright cut edges into a solution of 10 to 20 grams antimony (III) chloride in 1 liter of water. An alloy is considered suitable for coating with vitreous enamel if the corrosion has advanced under the enamel layer from the edge of the specimen less than 3 mm after at least 24 hours or, for more rigorous requirements, after 96 hours.
It is known that aluminum alloys which contain magnesium will not pass the test unless the sheet specimens before being enamelled have been subjected to the usual pretreatment by an alkaline degreasing treatment, possibly a pickling treatment, followed by a rinsing with water, and to an additional surface-passivating treatment with chromatecontaining solutions. For this purpose, a magnesiumfree alloy in accordance with U.S. Standard specification 3003, having a restricted tolerance range (less than 0.01% by weight magnesium) is used for the above-mentioned purpose.
That alloy has by weight the following composition:
1.0 to 1.3% manganese, PA1 0 to 0.4% silicon, PA1 0 to 0.6% iron, PA1 0 to 0.1% zinc, PA1 0.1 to 0.2% copper, PA1 balance aluminum. PA1 0.8 to 2.2% manganese, PA1 0.1 to 0.5% zirconium, PA1 0 to 1.0 % iron, PA1 0 to 0.6% silicon, PA1 0 to 0.5% copper PA1 0 to max. 0.1% magnesium,
After the above-mentioned test with antimony (III) chloride, this alloy does not exhibit any corrosion under the vitreous enamel layer. While the alloy has adequate strength, it has the disadvantage that it becomes soft as the vitreous enamel is baked, at temperatures above 500.degree. C, and that finished products made therefrom, such as pots and pans, have no satisfactory buckling strength.
AlMgSi alloys, e.g., Material No. 3.3206 in accordance with German Industrial Standard DIN 1717, are also known for such purposes and undergo a renewed precipitation hardening after the vitreous enamel has been baked so that the softening phenomenon is avoided. On the other hand, the use of these alloys is economically limited because their Mg content requires an expensive and ecologically unsatisfactory treatment with a chromate-containing solution.
It is also known that AlCuMg alloys in certain tolerance ranges can be coated with vitreous enamel without an additional pretreatment with chromate-containing solutions and that these alloys can be subjected to the desired precipitation hardening. Nevertheless, the alloys cannot be widely used because they do not resist intergranular corrosion, to which the finished products are normally subjet.
It has thus been found that these alloys can be provided with a vitreous enamel coating having a satisfactory bond strength but the pans, pots and the like are destroyed after a short time of use by an intergranular corrosion of the material.
Another object of the invention is to provide an improved method of fabricating particular structures with an aluminum alloy of high deformability, resistance to corrosion and recrystallization temperature.